There are well-known curable silicone rubber compositions which turn into silicone elastomers upon curing. Owing to their excellent weather resistance, heat resistance, and electrical insulating properties, they find general use as electric and electronic parts, electric wire coating, and automotive parts, whose typical examples include gasket material, potting material, coating material, roll material, and casting material for mold making.
However, they do not exhibit the silicone's outstanding properties, including chemical resistance (such as acid resistance and alkali resistance), water resistance, and gas permeability, under extremely severe conditions at high temperatures with high humidity, on account of the siloxane bond (which is the ionic bond) characteristic of the silicone.
New developments to cope with the foregoing disadvantage include polymers in which the siloxane bond is partly replaced by the silethylene bond or the silphenylene bond, as disclosed respectively in Patent Document 1 (WO 2001/030887) and Patent Document 2 (JP-A H05-320350). These polymers, however, have not yet been commercialized except for special uses and fields because of their difficulties in synthesis, problems with mass production, and high production cost.
It is known that a polydiarylsilmethylenesiloxane having the silmethylenesiloxane bond can be used as a precursor of silicon carbide-based ceramics, as disclosed in Patent Documents 3 to 5, JP-A H08-109264, JP-A H08-109265, and JP-A H08-109266. This polymer is a high-melting crystalline thermoplastic silicone polymer. It is excellent in heat resistance, insulation, electrical properties, chemical resistance, and water resistance but is poor in processability and moldability for practical use. Attempts have been made for its improvement through incorporation with a silicone polymer or incorporation with polydiarylsilmethylenesiloxane and polyalkylsilmethylenesiloxne, as disclosed respectively in Patent Document 6, JP-A H09-227781, and Patent Documents 7 and 8, JP-A H09-227782 and JP-A H09-227783. There is known a method for forming a film of polysilmethylenesiloxane on a substrate from a film of disilacyclobutane by ring opening polymerization with the help of a film of metal fine particles, as disclosed in Patent Document 9, Japanese Patent No. 3069655. However, the polydiarylsilmethylenesiloxane is a high-crystalline thermoplastic polymer but is not available in the form of thermosetting composition with the characteristic properties of polymer because of its difficulties in synthesis, high price and poor processability even though it was expected to be useful as a precursor for silicon carbide ceramics.